Surface Sensitive Atomic Force Microscope Based Infrared Spectroscopy

ABSTRACT

Systems and Methods may be provided for performing chemical spectroscopy on samples from the scale of nanometers with surface sensitivity even on very thick samples.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 USC § 1.119(e) to U.S.Provisional Patent Application Ser. No. 62/529,404, filed Jul. 7, 2017.The subject matter of this application is hereby incorporated byreference in its entirety.

BACKGROUND

The specification relates to scattering Atomic Force Microscope basedinfrared spectroscopy (AFM-IR) in particular for acquiring informationindicative of the distribution of chemical components in heterogeneoussystems.

AFM-IR may be a useful technique for measuring and mapping opticalproperties/material composition of some surfaces with resolutionapproaching nanometer scale. Various aspects of the technique aredescribed in U.S. Pat. Nos. 8,869,602, 8,680,457, 8,402,819, 8,001,830,9,134,341, 8,646,319, 8,242,448, and U.S. patent application Ser. Nos.13/135,956, 15/348,848, and 62/418,886 by common inventors and commonlyowned with this application. These applications are incorporated byreference in their entirety.

In the traditional AFM-IR technique, sample preparation can be achallenge. The technique relies on measuring the sample expansioncreated by absorption of the IR illumination inducing heating of thesample. This expansion generates motion in the AFM cantilever which ismeasured by typical AFM deflection detection techniques. The strength ofthe signal is dependent on a number of parameters related to the sampleand cantilever such as the spring constant of the cantilever, thethermal expansion of the sample and the penetration depth of the IRillumination. With a sample which has a thick absorbing region, theAFM-IR signal can be strong but the spatial resolution can degrade dueto heat diffusion within the sample leading to non-local (as defined bythe AFM tip) expansion. By preparing a sample as a thin layer on anon-absorbing or low thermal expansion substrate, the optimum spatialresolution can be achieved. This preparation can be accomplished using anumber of different sample preparation techniques, such as drop casting,spin coating, microtomy, etc. However, some types of samples do notallow usage of these preparation techniques, such as a thin organiccoating or layer on an organic substrate. In this type of sample the IRlight can penetrate beyond the thin top layer into the underlyingsubstrate. The IR light will then be absorbed in the substrate creatingexpansion which is an integration of the expansion over the fullpenetration depth of the IR illumination. This can mask the signal fromthe top layer. Moreover, this sample measurement can be especiallychallenging if the top layer is very thin (less than a few hundrednanometers) and/or has a chemistry which is similar to the substrate. Inthat case, the resultant signal may be dominated by the contributionfrom the thicker layer, with only a small modulation contributed by thethinner layer. As a result, an AFM-IR solution that accommodatesdistinguishing between layer of differing thickness (e.g., an organicthin layer and a thicker substrate, such as a thick polymer layer) wasdesired.

SUMMARY

Systems and Methods may be provided for performing chemical spectroscopyon samples from the scale of nanometers with surface sensitivity even onvery thick samples.

In order to address these above-noted drawbacks a new method has beendeveloped which has shown significantly better surface sensitivity. Thispatent describes the method and its benefits in any application wherethe top surface layer is of interest. This can include thin surfacecoatings or films, surface contamination or in composite materials nonuniform distribution of the materials relative to the depth from thesurface.

Definitions

“Optical property” refers to an optical property of a sample, includingbut not limited to index of refraction, absorption coefficient,reflectivity, transmissivity, transmittance, absorbance, absorptivity,real and/or imaginary components of the index refraction, real and/orimaginary components of the sample dielectric function and/or anyproperty that is mathematically derivable from one or more of theseoptical properties.

“Interacting a probe with a sample” refers to bringing the probe tipclose enough to the surface of a sample such that one or more near fieldinteractions occur, for example the attractive and/or repulsivetip-sample forces, and/or the generation and/or amplification ofradiation scattered from an area of the sample in proximity of the probeapex. The interaction can be contact mode, intermittent contact/tappingmode, non-contact mode, pulsed force mode, PeakForce Tapping® (PFT) modeand/or any lateral modulation mode. The interaction can be constant oras in some embodiments, periodic. The periodic interaction may besinusoidal or any arbitrary periodic waveform. Pulsed force modes and/orfast force curve techniques may also be used to periodically bring theprobe to a desired level of interaction with a sample, followed by ahold period, and then a subsequent probe retraction.

“Illuminating” means to direct radiation at an object, for example asurface of a sample, the probe tip, and/or the region of probe-sampleinteraction. Illumination may include radiation in the infraredwavelength range, visible, and other wavelengths from ultraviolet toTHz. Illumination may include any arbitrary configuration of radiationsources, reflecting elements, waveguiding elements such as fibers,focusing elements and any other beam steering or conditioning elements.

“Infrared light source” for the purposes of this specification refers toone or more optical sources that generate or emits radiation in theinfrared wavelength range. For example it can comprise wavelengthswithin the mid-IR (2-25 microns). An infrared light source may generateradiation over a large portion of these wavelength sub-regions, or havea tuning range that is a subset of one of the wavelength ranges, or mayprovide emission across multiple discrete wavelength ranges, for example2.5-4 microns, or 5-13 microns, for example. The radiation source may beone of a large number of sources, including thermal or Globar sources,laser-driven plasma sources, supercontinuum laser sources, frequencycombs, difference frequency generators, sum frequency generators,harmonic generators, optical parametric oscillators (OPOs), opticalparametric generators (OPGs), quantum cascade lasers (QCLs), nanosecond,picosecond, femtosecond and attosecond laser systems, CO2 lasers, heatedcantilever probes or other microscopic heaters, and/or any other sourcethat produces a beam of radiation, either in pulsed or in continuouswave operation. The source may be narrowband, for example with aspectral width of <10 cm⁻¹ or <1 cm⁻¹ or less, or may be broadband, forexample with a spectral width of >10 cm⁻¹, >100 cm⁻¹ or greater than 500cm⁻¹. “Near infrared light” generally refers to a wavelength range of IRlight corresponding to 0.75-2 μm.

“Signal indicative of” refers to a signal that is mathematically relatedto a property of interest. The signal may be an analog signal, a digitalsignal, and/or one or more numbers stored in a computer or other digitalelectronics. The signal may be a voltage, a current, or any other signalthat may be readily transduced and recorded. The signal may bemathematically identical to the property being measured, for exampleexplicitly an absolute phase signal or an absorption coefficient. It mayalso be a signal that is mathematically related to one or moreproperties of interest, for example including linear or other scaling,offsets, inversion, or even complex mathematical manipulations.

“Spectrum” refers to a measurement of one or more properties of a sampleas a function of wavelength or equivalently (and more commonly) as afunction of wavenumber.

“Infrared absorption spectrum” refers to a spectrum that is proportionalto the wavelength dependence of the infrared absorption coefficient,absorbance or similar indication of IR absorption properties of asample. An example of an infrared absorption spectrum is the absorptionmeasurement produced by a Fourier Transform Infrared spectrometer(FTIR), i.e. an FTIR absorption spectrum. (Note that IR absorptionspectra can also easily be derived from transmission spectra.)

“Modulating” or “modulation” when referring to radiation incident on asample refers to changing the infrared laser intensity at a locationperiodically. Modulating the light beam intensity can be achieved bymeans of mechanical chopping of the beam, controlled laser pulsing,and/or deflecting the laser beam, for example by a tilting mirror thatis driven electrostatically, electromagnetically, with piezo actuatorsor other means to tilt or deform the mirror, or high speed rotatingmirror devices. Modulation can also be accomplished with devices thatprovide time varying transmission like acousto-optic modulators,electro-optic modulators, photo-elastic modulators, pockel cells, andthe like. Modulation can also be accomplished with diffraction effects,for example by diffractive MEMS-based modulators, or by high speedshutters, attenuators, or other mechanisms that change the intensity,angle, and/or phase of the laser intensity incident on the sample.

“Demodulate” or “demodulation” refers to extracting aninformation-bearing signal from an overall signal, usually, but notnecessarily at a specific frequency. For example in this application,the collected probe light collected at a photo detector represents anoverall signal. The demodulation process picks out the portion that isbeing perturbed by infrared light absorbed by the sample. Demodulationcan be accomplished by a lock-in amplifier, a fast Fourier transform(FFT), a calculation of a discrete Fourier component at a desiredfrequency, a resonant amplifier, a narrow band bandpass filter, or anyother technique that largely enhances the signal of interest whilesuppressing background and noise signals that are not in sync with themodulation. A “demodulator” refers to a device or system that performsdemodulation.

A “analyzer/controller” refers to a system to facilitate dataacquisition and control of the system. The controller may be a singleintegrated electronic enclosure or may comprise multiple distributedelements. The control elements may provide control for positioningand/or scanning of the probe tip and/or sample. They may also collectdata about the probe deflection, motion or other response, providecontrol over the radiation source power, polarization, steering, focusand/or other functions. The control elements etc. may include a computerprogram method or a digital logic method and may be implemented usingany combination of a variety of computing devices (computers, PersonalElectronic Devices), analog and/or digital discrete circuit components(transistors, resistors, capacitors, inductors, diodes, etc.),programmable logic, microprocessors, microcontrollers,application-specific integrated circuits, or other circuit elements.Memory elements configured to store computer programs, which may executefrom memory, may be implemented along with discrete circuit componentsto carry out one or more of the processes described herein.

A “lock-in amplifier” is one example of a “demodulator” (defined above)and is a device, system, and/or an algorithm that demodulates theresponse of a system at one of more reference frequencies. Lock-inamplifiers may be electronic assemblies that comprise analogelectronics, digital electronics, and combinations of the two. They mayalso be computational algorithms implemented on digital electronicdevices like microprocessors, field programmable gate arrays (FPGAs),digital signal processors, and personal computers. A lock-in amplifiercan produce signals indicative of various metrics of an oscillatorysystem, including amplitude, phase, in phase (X) and quadrature (Y)components or any combination of the above. The lock-in amplifier inthis context can also produce such measurements at both the referencefrequencies, higher harmonics of the reference frequencies, and/orsideband frequencies of the reference frequencies.

“Optical response” refers to the result of interaction of radiation witha sample. The optical response is related to one or more opticalproperties defined above. The optical response can be an absorption ofradiation, a temperature increase, a thermal expansion, a photo-inducedforce, the reflection and/or scattering of light, a phase transition, orother response of a material due to the interaction with radiation.

“Sideband frequency” refers to a frequency that is a linear sum ordifference of two excitation frequencies. For example, if a system isexcited at frequencies f₁ and f₂, a sideband frequency can be anyfrequency f_(sb) that satisfies f_(sb)=|±f₁±f₂|. More generally, in somecases a sideband frequency can also be a linear sum or difference of oneof more harmonics of the excitation frequencies, i.e. f_(sb)=|±mf₁±nf₂|,where m and n are integers.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects and advantages of the embodiments provided herein are describedwith reference to the following detailed description in conjunction withthe accompanying drawings. Throughout the drawings, reference numbersmay be re-used to indicate correspondence between referenced elements.The drawings are provided to illustrate example embodiments describedherein and are not intended to limit the scope of the disclosure.

FIGS. 1A and 1B show simplified schematic diagrams of an illustrativeembodiment including capability to perform surface sensitive AFM-IRmeasurements.

FIGS. 2A and 2B illustrate a mechanism by which absorption of IRradiation in the sample can result in a shift in a resonance frequencyof an AFM probe.

FIGS. 3A and 3B show an example measurement of a surface sensitiveAFM-IR measurement on a sample of a thin layer of nylon on a thick filmof polyethylene terephthalate PET.

FIG. 4 illustrates how a heating induced change in AFM probe resonancefrequency can result in an associated change in the probe's oscillationphase.

FIG. 5 shows a simplified schematic diagram of an embodiment of surfacesensitive AFM-IR.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The current disclosure is directed towards obtaining measurements ofinfrared optical properties of a material on a length scale much, muchsmaller than the diffraction limit of the infrared wavelengths employed,and in fact down to the nanometer scale, with the ability to performsurface sensitive IR spectroscopy, even on thick samples with strong IRabsorption. This is achieved by the use of a contrast mechanism that ishighly sensitive to the temperature of the sample surface.

The current disclosure describes a method and apparatus for obtaininghigh sensitivity IR absorption spectra from nanoscale regions of asample with a high degree of surface sensitivity that is simultaneouslyhighly insensitive to IR absorption of sub-surface layers, includingvery thick layers. Referring to FIG. 1A, a beam of IR radiation 100 isfocused onto a region of a thin sample of interest 102 on a much thickersubstrate 104. It is desirable to measure the IR absorption of the thinregion 102 without having the measurement heavily contaminated by the IRabsorption of the underlying substrate 104. To measure the IR absorptionproperties of the thin sample 106, the apex 106 of an AFM probe 108 isbrought into interaction with the surface of thin sample 106. As theprobe apex 106 interacts with the surface of thin sample 102, thecantilever 108 will exhibit one or more resonant frequencies, forexample as indicated with curve 110 in FIG. 1B. These resonances may beresonances associated with static contact between the probe apex andsample (so called contact resonances) or may be associated withresonances of the cantilever in dynamic operation, e.g. tapping,intermittent contact or other schemes involving periodic interactingbetween the tip and sample. All of these resonance frequencies can shiftdue to a change in temperature of the sample. In the current embodiment,when the IR beam 100 is tuned to an absorption of the thin sample 102 inFIG. 1A, the absorbed IR radiation turns into heat, raising thetemperature of the sample 102. This temperature increase ΔT can cause aresulting shift in the cantilever resonant frequency. For example, theresonance 110 in FIG. 1B may shift to a lower frequency, as shown incurve 112. The temperature change ΔT can thus result in a resonancefrequency shift Δf. This resonance frequency shift can then be measuredas a function of wavelength of the IR beam 100 to construct an IRabsorption spectrum for the region of sample 102 in contact with theprobe apex 106. The AFM probe may locally enhance the electric fieldproduced by the incident IR beam 100, resulting in a larger localtemperature increase in the surface layer just under the tip apex. Thiscan also help magnify the contribution of the relative impact of thesurface layer versus that of a substrate or other buried layers that arefarther from the field enhancing tip apex. Although AFM cantileverprobes were used to describe the above embodiment, the surface sensitivetechnique described above can also be applied to other forms of scanningprobes, for example tuning fork probes, or MEMS devices with sharpprobes attached, as long as an interaction between the tip and samplecan result in a shift in one or more resonances of the device.

FIGS. 2A and 2B show one mechanism that can result in temperaturedependent changes in probe resonant frequency. FIG. 2A shows a diagramshowing the typical dependence of elasticity (E) as a function oftemperature and the corresponding material states, glassy, rubbery andliquid for many polymeric materials. Although the detailed behavior maybe quite complex over large temperature ranges and different materialphases, over any small temperature range, there is usually a moderatechange in elastic modulus with temperature. The scale of the elasticmodulus change can vary dramatically, depending on whether the materialis in a glassy, rubber or liquid regime, but in all regimes, there is achange in elasticity with temperature. So as the sample absorbs IR lightand heats up an amount ΔT, there is a resulting change ΔE in the elasticproperties of the sample. (There can also be changes in viscoelasticproperties, including changes in material damping/dissipation as well.)Turning to FIG. 2B, this change in elasticity of the sample can have acommensurate effect on the resonance of the AFM cantilever probe. Theviscoelasticity of the probe sample contact acts as spring/dashpotsystem that affects the resonance frequency of the cantilever probe whenthe probe apex is in interaction with the surface of the sample. So asthe elastic modulus E of the sample changes an amount ΔE, the resonancefrequency can change as well an amount Δf. So for a given amount of IRabsorption, the result is a temperature increase, which generallyresults in an elasticity decrease, and this will generally lead to adecrease in the resonance of the cantilever probe while interacting withthe sample. So by measuring the probe resonance frequency or by proxymeasuring the cantilever oscillation phase, it is possible to generate asignal that is indicative of the IR absorption of the region of thesample. And, advantageously, this resonance frequency is only sensitiveto the surface or near surface of the sample and is minimally sensitiveto elasticity of material well below the sample surface. Because ofthis, it is possible to obtain IR absorption spectra on very thicksamples without substantial contamination of the absorption ofunderlying layers.

The reason that this technique is so highly surface sensitive is thatthe changes in resonant frequency of the AFM cantilever probe resultonly from the interactions within a short distance from the apex of theAFM probe. That is, changes in resonant frequency of the AFM cantileverare dependent on the viscoelastic properties of the sample material onlywithin a few nanometers or tens of nanometers of the AFM probe tip. TheAFM cantilever resonance is thus highly insensitive to the viscoelasticproperties of material deep within a sample. Specifically, thecantilever resonance is typically sensitive to the material propertiesvery close to the surface, for example in some cases of the sample oforder of the diameter of the contact area between the probe tip apex andsample. The probe contact can be on the scale of a few nanometers to afew tens of nanometers across, depending on the probe sharpness, contactforce, and sample viscoelasticity.

Note that other temperature dependent properties can also affect thecantilever resonant frequency and/or phase. For example, the temperatureincrease can also cause changes in friction, adhesion, and damping. Anyof these properties can cause a shift in the probe resonance frequency.Additionally, thermal expansion of the sample and/or tip that can causechanges in the probe loading force that can also shift the proberesonance frequency and/or phase. Note for this technique to work, it isnot strictly necessary to know the exact mechanism that is dominant. Itis sufficient that a change in temperature in the surface of the samplecauses a measurable change in the properties of the sample that cancause a phase or frequency change in the probe. As will be discussedlater, it is also possible to measure and correct for any nonlinearitiesin the specific mechanism that converts temperature change into resonantfrequency and/or phase change.

FIGS. 3A and 3B show an example measurement of a surface sensitiveAFM-IR measurement on a sample of a thin layer of nylon on a thick filmof polyethylene terephthalate (PET). FIG. 3A shows an optical microscopeimage 300 of a region of a sample that comprises a thick layer of PET,250 micrometers thick, with a thin layer of nylon on top. The nylonlayer has been microtomed to a thickness of around 300 nm and depositedon the much thicker PET. FIG. 3B shows AFM-IR spectra obtained under thecurrent embodiment at regions 302 and 304 shown in the opticalmicroscope image 300 in FIG. 3A. The spectra were obtained by measuringthe phase shift of a contact mode resonance of an AFM cantilever whilesweeping the wavenumber (or equivalently wavelength) of the IR laserilluminating the sample. As the contact mode resonance changes with theamount of absorbed light, the effect of this resonance change can berecorded quickly by measuring the phase change. The reason for this isthat if the cantilever is driven into oscillation at a fixed frequency,if the cantilever resonance frequency changes due to the absorbed IRlight (and resulting temperature increase and change in samplemechanical properties), there will be a corresponding change in phase asexplained in association with FIG. 4 below. The key thing to note inFIG. 3B is that the absorption peaks 310 and 312 that are associatedwith IR absorption of the nylon are about 5× bigger than the peak 314which is the residual effect of the carbonyl peak from the underlyingPET material. So the nylon absorption bands are 5× bigger than theresidual PET peak, despite the fact that the PET film is more than 800×thicker than the nylon layer (250,000 nm versus 300 nm). The residualcarbonyl peak 314 from the PET in spectrum 308 is also attenuated byabout 8× from the measurement 306 directly on the PET. This plotdemonstrates the ability of the current embodiment to perform “thin onthick” measurements, i.e. to perform chemical analysis of a thin surfacelayer, despite the large potential background of IR absorption of a muchthicker underlayer. And specifically, we have demonstrated the abilityto obtain IR spectra on surface layers on thick films that are more than800× thicker than the surface film. This represents a breakthrough inthe ability to measure samples like surface coatings on thicksubstrates.

The spectra in FIG. 3B comprised measurements of phase of the cantileveroscillation as a function of wavenumber of the IR source. The followingdiscussion explains why the phase signal can be used as a signalindicative of the IR absorption. An example of the relationship betweena cantilever resonance and the cantilever phase is shown in FIG. 4. Plot400 shows two cantilever resonances, 404 with no IR absorption and 406when the IR absorption has caused a temperature induced shift in theresonance peak. The corresponding phase curves are shown in plot 402,with phase curve 408 corresponding to the case with no absorption andplot 410 after the resonance frequency has shifted due to the IRabsorption induced temperature change. If the system is driven intooscillation at a fixed frequency 412, there will be a correspondingchange in phase Δφ (414) associated with the IR absorption. Thusplotting this phase signal as a function of the wavelength or wavenumberof the IR source results in a localized IR absorption spectrum. Becausethe position of this resonance is only dependent on the mechanicalproperties of the surface material within a small distance of the probeapex, we can obtain a highly surface sensitive measurement of the IRabsorption.

The phase or frequency signal can be measured in one or more ways. Forthe measurement shown in FIG. 3, the phase was measured by modulatingthe IR light at a frequency near a contact resonance of the cantilever.In can also be performed measuring at a tapping/intermittent contactresonance and/or a sideband frequency. Additionally, the phase orresonant frequency does not need to be measured with the modulation ofthe IR beam. Alternately, the resonance frequency and/or phase can bemeasured with mechanical excitation of the cantilever, for example witha piezoelectric element, or electrostatically, magnetically, viaultrasound, thermal bending, via bimetallic bending, via photothermalexcitation of the cantilever, or other methods to drive the cantileverinto oscillation. It can be advantageous, for example, to modulate theIR source at very high frequencies to create a short thermal diffusionlength, but then measure the cantilever resonance properties at adifferent frequency that provides more sensitive detection of thecantilever phase and/or frequency.

Although the relationship between phase shift and frequency shift isroughly linear for small frequency shifts and small phase shifts, it isnon-linear for larger frequency shifts. It may be desirable to convertthe phase spectra into equivalent absorption spectra by scaling thephase curves by the nonlinear relationship between phase and frequency.Alternately, the whole data chain can be linearized if necessary, i.e.correcting for nonlinearities in frequency shift with sample elasticityand sample elasticity with temperature. This linearization process canbe performed by first performing a calibration sequence, for examplemeasuring the change in elasticity of a sample as a function oftemperature, by raising the temperature of the sample by a known amountor in known increments. Similarly, the nonlinearity of the change in thecantilever resonance with sample elasticity can be calibrated onreference samples with known elastic modulus, or again by measuring thechange in contact resonance as a function of known sample temperature.Under certain conditions, the expected nonlinear dependence can bycalculated analytically or by finite element methods. Publications byRabe, Stark and others provide analytical expressions for shift inresonance frequencies in interaction with a sample surface (for examplesee Rabe, U.; Arnold, W.; Janser, K., Vibrations of free andsurface-coupled atomic force microscope cantilevers. Theory andexperiment. 1996; and Rabe, U.; Kopycinska, M.; Hirsekorn, S.; Arnold,W., Evaluation of the contact resonance frequencies in atomic forcemicroscopy as a method for surface characterisation (invited).Ultrasonics 2002, 40 (1-8), 49-54; and Stark, R.; Schitter, G.; Stark,M.; Guckenberger, R.; Stemmer, A., State-space model of freely vibratingand surface-coupled cantilever dynamics in atomic force microscopy.Phys. Rev. B 2004, 69 (8).) The nonlinear shift of phase with resonancefrequency shift can also be calibrated for example by interacting theprobe with different samples, or samples at different temperatures,and/or interacting the probes with the sample at different interactionforces. Additionally it is possible to apply analytical or finiteelement models, for example simple harmonic oscillatory models, EulerBernoulli type models or other analytical techniques. Applying thesetechniques to linearize the spectra can be highly desirable in casesthat the absorption bands of interest occur over a large dynamic range,for example ranging from very strong absorption bands to very weakbands. In these cases, linearizing the response can be desirable toenable better matching between AFM-IR spectra and materials spectraldatabases and/or for quantification of material concentrations. But forsmall increases in temperature, e.g. of order a few degrees or less,this linearization may not be necessary, especially away fromtemperatures where the sample surface undergoes a material transition,e.g. a glass transition or melt.

In another embodiment it is also possible to linearize the response bymeasuring the probe response as a function of the power of the incidentIR beam on the sample. At a given absorption band, the temperatureincrease in the sample varies linearly with incident power of the IRbeam. By varying the IR beam power and measuring the resulting proberesponse (e.g. the probe resonance frequency and/or phase) it ispossible to directly construct a calibration between temperature andphase or frequency. This provides a one-step process for calibrating allof the nonlinearity between sample temperature and measured proberesponse.

Note that it is also possible to automatically select a region withsufficiently low amounts of non-linearity. Specifically, the laser powercan be adjusted over a range of power to simulate the likely dynamicrange of the temperature increases in the sample over a plurality ofabsorption bands. If the probe response is sufficiently linear over therange of IR laser power used, it may not be necessary to linearize theprobe response to generate a signal that is adequately indicative of theIR absorption of the surface layer. Performing this test can also helpavoid power levels that could provoke a nonlinear material response,e.g. resulting from the material temperature rising enough to go througha softening transition, for example a glass to rubber transition or amelt.

FIG. 5 shows a schematic diagram of an embodiment of a surface sensitiveAFM-IR system. A probe tip 502 of a scanning probe microscope isinteracted with a region 506 of a sample 504. In one embodiment, theprobe comprises a cantilever 500 that is oscillated by an actuator 510at at least one frequency f₁ driven by signal generator 512 The actuatoris most commonly a piezoelectric element, but it can also comprisealternate drive mechanisms including magnetic, electrostatic, thermal,optical force or other schemes that apply an oscillatory force on thecantilever to drive it into oscillation. In one embodiment the frequencyf₁ may be selected to correspond to a resonance of cantilever 500, butin other embodiments this is not necessary. A beam of infrared radiation518 from an infrared light source 514 is used to illuminate a sample 504in the vicinity of the tip 502 and the region of interest of the sample506. In one embodiment, the probe response is measured via a deflectiondetection system 520, for example an optical lever system used tomeasure position, deflection, bend, and/or motion of the cantileverprobe.

It can be desirable to discriminate changes in the probe response fromIR absorption (the signal of interest) from parasitic effects that cancause shifts in the probe response. For example, cantilever resonancefrequency and/or phase can also change due to environmental temperaturechanges, changes in humidity, drift in the tracking force, change in thecontact area, and other factors such as heating and possiblebending/distortion of the cantilever itself during IR illumination. Toeliminate these issues from causing noise in the spectra, it can bedesirable to do a differential measurement. For example, it is possibleto rapidly compare the probe response with the IR source on and off.Measurements with the IR source off can provide information about driftsin the cantilever frequency/phase shift that are independent of thesample temperature rise due to IR absorption. It is also possible topulse or modulate the IR source at one frequency and drive thecantilever into oscillation at another frequency with an alternativedrive scheme, for example a piezo actuator or one or more of the othermethods described elsewhere. In this case it is possible to use thenon-laser driven resonance to track drifts in the probe resonancefrequency/phase when the laser is not pulsing or not on. Note that thisneed not add any measurement time to the measurement process. Forexample, many pulsed IR sources operate at low duty cycle, a few percentor less. So there is ample time during the time between pulses toperform a separate measurement of the probe frequency/phase while thelaser is off and the sample is relaxed back to a default thermal state.This baseline response can then be subtracted from the probe responsemeasured as a function of wavelength/wavenumber with the IR laserpulsing.

The illumination system may include any number of lenses, mirrors,attenuators, polarizers, pinholes, waveguides (fibers), beam steeringelements to direct and condition the beam prior to arriving at thetip-sample region. In general, the light is focused to a spot, althoughin general the focused light spot is much larger than the tip-sampleinteraction region. The focusing optics may include lenses and/orreflective focusing elements, for example parabolic mirrors, includingoff axis parabolic mirrors. The light, however, is often further“nanofocused” and/or intensified by the probe tip geometry and/orsurface coating leading to an intensification of the electric field feltat the sample as a result of the incident radiation.

The radiation incident on the sample may interact with the sample andproduce a detectable response. For example, if the wavelength of the IRradiation is tuned to an absorption band of the sample material, aportion of the incident radiation will be absorbed. The absorbedradiation can cause heating of the sample region, in turn resulting in atemperature rise and a thermal expansion of the absorbing region, butalso a temperature dependent shift in the mechanical properties of thesample, as described previously. The incident radiation may also inducea force on the probe tip, either through the thermal expansion and/orthrough interactions of the electric field of the probe and the electricfield of the sample, and/or it may cause a shift in a resonant frequencyof the AFM probe. In any case, a probe response can be measured inresponse to the radiation incident on the sample by one or moredetection systems in the scanning probe microscope. The probe responsecan be elicited by measuring a temperature rise in the probe, adeflection, oscillation or force on the probe, one or more resonantfrequencies, and/or oscillation phases of the probe. By changing thewavelength emitted from the radiation source to a wavelength absorbed byanother material component, it is possible to map the distribution ofthat component. Measuring the probe response at a plurality ofwavelengths (or equivalently wavenumbers) will result in a spectrum thatis representative of the optical response of the sample, or in specificcases an IR absorption spectrum.

In one embodiment, the radiation beam 518 is modulated at least onefrequency f_(m). This modulation may comprise an intensity modulation,an angle modulation or other modulation that creates a periodicvariation in the strength of the radiation incident on the sample in thevicinity of the probe tip. The modulation may comprise a series ofpulses or may be sinusoidal in nature or other arbitrary waveform shapewith a periodic component at frequency f_(m). In the case of a pulsedsource, the modulation frequency f_(m) can refer to the pulse repetitionrate of the pulsed source. In one embodiment, the modulation may beaccomplished for example by providing a modulation signal, a gatingpulse, an external trigger or sync pulse to light source 514 thatelectronically modulates the intensity of the beam of radiation.Alternately, this modulation may be accomplished via an externalmodulator, for example a chopper, an electrooptic modulator, anelectroacoustic modulator, a photoelastic modulator, an electronicshutter, a MEMS mirror, a high speed galvo, a piezo driven mirror or anyother device that can periodically adjust the intensity and/or angle ofa light beam that passes through the modulator. The light source mayalso be modulated by providing an analog modulation signal, for exampleto modulate the voltage and/or current provided to a light source, forexample a quantum cascade laser.

In a specific embodiment a lock-in amplifier 522 can measure theoscillatory response of the probe 500, for example the amplitude Aand/or phase □ of the probe at a one or more frequencies f_(i),including the modulation frequency and/or one or more sidebandfrequencies. A controller 524 can read in data from the deflectiondetector 520, the lock-in amplifier 522 and other auxiliary signals asdesired. The controller 524 can also output pulses 516 to control themodulation of light source 514 or to an external modulator. Alternatelyit can simply send analog or digital commands to change the modulationrate of the light source. Controller 524 can also control the positionof scanner 526 to control the relative tip/sample position. It can alsobe used to adjust any of the probe interaction parameters including theoscillation frequency (or frequencies) and amplitude(s) of the probe,the amplitude setpoint, scan speed parameters, feedback parameters, etc.It is understood that such a system includes one or more processingelements, shown as controller 524, but may in fact be distributed amonga variety of processing elements including any combination of digitallogic and/or computing devices connected to some or all of variousactuators, sensors and user interface elements, displays, output devicesand networks, wired and/or wireless. The system actions, dataacquisition, and data processing described in this disclosure, in manycases, are the result of logical sequences and/or computerprograms/applications executing form memory on the processing elements.

Controller 524 can also provide computation and analysis on any of theinput signals to produce a compositional maps and/or spectra 528 basedon the measured probe response. The compositional map is a map of thedistribution of one or more material components in a heterogeneoussample. At any position on the sample it is also possible to obtain anIR absorption spectrum (i.e. measurements of the probe response as afunction of wavelength or wavenumber). In the case that surfacesensitive spectra are desired, the system can be used to measure aresonant frequency of the probe in interaction with the sample surfaceand/or the phase of the cantilever resonance. This frequency and/orphase as a function or the IR wavelength or wavenumber can generatesurface sensitive spectra as shown in FIG. 3.

Note that there are multiple ways that the cantilever resonancefrequency and/or phase can be determined. In one embodiment, thefrequency and/or phase can be determined by pulsing or modulating the IRsource 514 at a frequency f_(m) that is substantially near a resonancefrequency of the AFM probe. In this case, the IR absorption of thesample and/or probe can induce a force on the probe tip to drive it intooscillation. Then the detector 520 can record the oscillatory responseof the cantilever 500 to determine resonance frequencies and/or theoscillation phase. It is alternatively possible to use other means todrive the cantilever into oscillation to measure the cantileverresonance and/or phase. For example a piezo actuator 510 may be drivenby oscillation drive 512 at or near a cantilever resonance. As mentionedpreviously, alternative drive schemes may also be employed, for exampleelectrostatic, magnetic, ultrasound, thermal bending, bimetallicbending, photothermal excitation, or other techniques that can apply anoscillatory force to the cantilever. A phase locked look can beimplemented to dynamically track the temperature dependent changes inthe resonance frequency. Thus local absorption spectra can be created bymeasuring the change in resonance frequency as a function of thewavelength or wavenumber of the incident radiation.

In one embodiment the probe response is detected at a “sidebandfrequency” that results from the nonlinear mixing of forces in theregion of tip sample interaction that results in the generation of forcecomponents at sum and difference frequencies of the frequencies of tipand sample excitation. More specifically if the cantilever is oscillatedat a frequency f₁ and the radiation incident on the sample is modulatedat frequency f_(m), in the presence of a non-linear mixing force, therewill be frequency components at “sideband frequencies” f_(sb), i.e. sumand difference frequencies, where f_(sb)=|±f₁±f_(m)|. (Or more generallylinear combinations of any integer harmonics of these frequencies.)

The presence of probe response at sideband frequencies can come about bythe following process. Consider a situation in which the tip-sampleforce has both linear and nonlinear terms based on the relativetip-sample separation. For example, to just quadratic terms, the tipsample force may be written as:

F _(ts) =k(z _(s) z _(t))+γ(z _(s) z _(t))²;  Eq. 1:

where k_(s) is the sample's linear contact stiffness, z_(s) and z_(t)are the sample position and the tip position respectively. The samplemotion term z_(s) is wavelength dependent and contains information aboutthe sample's optical properties and/or IR absorption. The gamma term isthe constant of proportionality to any quadratic dependence of thetip-sample force on tip-sample separation and as such is a term that isindicative of a nonlinear tip-sample interaction. (It is alsoproportional to the 2^(nd) derivative of the tip sample force withseparation.) If the motions of the tip and sample are periodic, theterms z_(s) and z_(t) will have Fourier components:

Z _(s1) =a _(s) cos(2πf _(m) t) and  Eq. 2:

Z _(t1) =a _(t) cos(2πf _(1t)+φ_(ts));  Eq. 3:

where a_(s) and a_(t) are the Fourier components of the tip and samplemotion at the modulation frequency f_(m) and the tip oscillationfrequency f₁ respectively, and φ_(ts) is the relative phase between thetip and sample motions. (If the motion of the tip and sample arenon-sinusoidal, there will also be other Fourier components at higherharmonic frequencies, but we will omit them for simplicity in thecurrent discussion.)

If we plug the values of z_(s1) and z_(t1) into Eq. 1 for z_(s) andz_(t), the quadratic term will be:

F _(ts2)=γ(Z _(s1) Z _(t1))²=γ(a _(s) cos(2πf _(mt))a _(t) cos(2πf_(1t)+φ_(ts)))²  Eq. 4:

When multiplied out, Eq. 4 the tip-sample force will contains across-term F_(ts) _(_) _(sb):

F _(ts) _(_) _(sb)=2γa _(s) a _(t) cos(2πf _(mt))cos(2πf1t+φ _(ts))  Eq.5:

This multiplication of the two cosines creates cross-terms (i.e. beatresponses) at sum and difference frequencies of the tip and samplemotion, i.e. at sideband frequencies f_(sb):

f _(sb) =|±f ₁ ±f _(m)|  Eq. 6:

If f_(sb) is selected to correspond to a resonance of the cantilever,the detected amplitude will be preferentially enhanced. Alternately, ifthe frequencies of f1 and f_(m) are fixed, but the sample's materialproperties change due to a sample temperature increase due to IRabsorption, then the cantilever resonances will change. Thus measuringthe shifts in the sideband frequency can produce a signal that isindicative of the IR absorption of the surface layer of the sample.Alternately or additionally, if the phase of the sideband frequency ismeasured as a function of IR illumination wavelength, it is possible toconstruct a spectrum of IR absorption of the surface layer of thesample.

The embodiments described herein are exemplary. Modifications,rearrangements, substitute processes, alternative elements, etc. may bemade to these embodiments and still be encompassed within the teachingsset forth herein. One or more of the steps, processes, or methodsdescribed herein may be carried out by one or more processing and/ordigital devices, suitably programmed.

Depending on the embodiment, certain acts, events, or functions of anyof the method steps described herein can be performed in a differentsequence, can be added, merged, or left out altogether (e.g., not alldescribed acts or events are necessary for the practice of thealgorithm). Moreover, in certain embodiments, acts or events can beperformed concurrently, rather than sequentially.

The various illustrative logical blocks, optical and control elements,and method steps described in connection with the embodiments disclosedherein can be implemented as electronic hardware, computer software, orcombinations of both. To clearly illustrate this interchangeability ofhardware and software, various illustrative components, blocks, modules,and steps have been described above generally in terms of theirfunctionality. Whether such functionality is implemented as hardware orsoftware depends upon the particular application and design constraintsimposed on the overall system. The described functionality can beimplemented in varying ways for each particular application, but suchimplementation decisions should not be interpreted as causing adeparture from the scope of the disclosure.

The various illustrative logical blocks and modules described inconnection with the embodiments disclosed herein can be implemented orperformed by a machine, such as a processor configured with specificinstructions, a digital signal processor (DSP), an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA) orother programmable logic device, discrete gate or transistor logic,discrete hardware components, or any combination thereof designed toperform the functions described herein. A processor can be amicroprocessor, but in the alternative, the processor can be acontroller, microcontroller, or state machine, combinations of the same,or the like. A processor can also be implemented as a combination ofcomputing devices, e.g., a combination of a DSP and a microprocessor, aplurality of microprocessors, one or more microprocessors in conjunctionwith a DSP core, or any other such configuration.

The elements of a method, process, or algorithm described in connectionwith the embodiments disclosed herein can be embodied directly inhardware, in a software module executed by a processor, or in acombination of the two. A software module can reside in RAM memory,flash memory, ROM memory, EPROM memory, EEPROM memory, registers, harddisk, a removable disk, a CD-ROM, or any other form of computer-readablestorage medium known in the art. An exemplary storage medium can becoupled to the processor such that the processor can read informationfrom, and write information to, the storage medium. In the alternative,the storage medium can be integral to the processor. The processor andthe storage medium can reside in an ASIC. A software module can comprisecomputer-executable instructions which cause a hardware processor toexecute the computer-executable instructions.

Conditional language used herein, such as, among others, “can,” “might,”“may,” “e.g.,” and the like, unless specifically stated otherwise, orotherwise understood within the context as used, is generally intendedto convey that certain embodiments include, while other embodiments donot include, certain features, elements and/or states. Thus, suchconditional language is not generally intended to imply that features,elements and/or states are in any way required for one or moreembodiments or that one or more embodiments necessarily include logicfor deciding, with or without author input or prompting, whether thesefeatures, elements and/or states are included or are to be performed inany particular embodiment. The terms “comprising,” “including,”“having,” “involving,” and the like are synonymous and are usedinclusively, in an open-ended fashion, and do not exclude additionalelements, features, acts, operations, and so forth. Also, the term “or”is used in its inclusive sense (and not in its exclusive sense) so thatwhen used, for example, to connect a list of elements, the term “or”means one, some, or all of the elements in the list.

Disjunctive language such as the phrase “at least one of X, Y or Z,”unless specifically stated otherwise, is otherwise understood with thecontext as used in general to present that an item, term, etc., may beeither X, Y or Z, or any combination thereof (e.g., X, Y and/or Z).Thus, such disjunctive language is not generally intended to, and shouldnot, imply that certain embodiments require at least one of X, at leastone of Y or at least one of Z to each be present.

The terms “about” or “approximate” and the like are synonymous and areused to indicate that the value modified by the term has an understoodrange associated with it, where the range can be ±20%, ±15%, ±10%, ±5%,or ±1%. The term “substantially” is used to indicate that a result(e.g., measurement value) is close to a targeted value, where close canmean, for example, the result is within 80% of the value, within 90% ofthe value, within 95% of the value, or within 99% of the value.

Unless otherwise explicitly stated, articles such as “a” or “an” shouldgenerally be interpreted to include one or more described items.Accordingly, phrases such as “a device configured to” are intended toinclude one or more recited devices. Such one or more recited devicescan also be collectively configured to carry out the stated recitations.For example, “a processor configured to carry out recitations A, B andC” can include a first processor configured to carry out recitation Aworking in conjunction with a second processor configured to carry outrecitations B and C.

While the above detailed description has shown, described, and pointedout novel features as applied to illustrative embodiments, it will beunderstood that various omissions, substitutions, and changes in theform and details of the devices or methods illustrated can be madewithout departing from the spirit of the disclosure. As will berecognized, certain embodiments described herein can be embodied withina form that does not provide all of the features and benefits set forthherein, as some features can be used or practiced separately fromothers. All changes which come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

We claim:
 1. A method of obtaining a signal indicative of an infraredabsorption spectrum of a thin surface layer of a sample with a probe ofa scanning probe microscope, comprising the steps of: a. Interacting theprobe with a surface layer of the sample; b. Illuminating the surfacelayer with a beam of infrared radiation; c. Measuring a probe responsecomprising at least one of a resonance frequency shift and a phase shiftof a resonance of the probe in response to infrared radiation absorbedby the surface layer; d. Measuring the probe response at a plurality ofwavelengths of the infrared radiation; e. Constructing a signalindicative of infrared absorption of the surface layer.
 2. The method ofclaim 1 wherein the surface layer is disposed on a substrate that alsoabsorbs IR radiation over the plurality of wavelengths.
 3. The method ofclaim 2 wherein the substrate is at least 10× thicker than the surfacelayer.
 4. The method of claim 2 wherein the substrate is at least 100×thicker than the surface layer.
 5. The method of claim 2 wherein thesubstrate is at least 800× thicker than the surface layer.
 6. The methodof claim 2 wherein the signal indicative of the IR absorption of thesurface layer is at least 5× stronger than residual absorption bandsfrom the substrate.
 7. The method of claim 2 wherein residual absorptionpeaks from the substrate that appear in the signal indicative of the IRabsorption of the surface layer are at least 8× smaller than absorptionpeaks measured on bare substrate without the surface layer. The methodof claim 1 wherein the measuring probe response step comprisesmodulating an intensity of the beam of infrared radiation to induce anoscillatory response of the probe at or near a resonance of the probe.8. The method of claim 1 wherein the probe is oscillated at a frequencywherein there is a substantially maximum slope of probe oscillationphase with resonance frequency shift.
 9. The method of claim 1 whereinthe measuring probe response step comprises oscillating the probe at ornear a resonance of the probe with a piezoelectric actuator.
 10. Themethod of claim 1 wherein the measuring probe response step comprisesmodulating the probe at or near a resonance of the probe with anactuator comprising at least one of: an electrostatic drive, a magneticdrive, an acoustic drive, an ultrasonic drive, a photothermal drive, abimetallic drive, and joule heating thermal drive.
 11. The method ofclaim 1 further comprising the step of linearizing the signal indicativeof infrared absorption of the surface layer.
 12. The method of claim 12wherein the linearizing step comprises compensating for at least one of:nonlinear dependence of sample elasticity with temperature, nonlineardependence of probe resonant frequency with sample elasticity, andnonlinear dependence of probe oscillation phase with resonance frequencyshift.
 13. The method of claim 12 wherein the linearizing step comprisesmeasuring a probe response as a function of power of the beam ofinfrared radiation to create a linearization function.
 14. The method ofclaim 14 wherein measuring probe response as a function of IR beam poweris used to infer a relationship between probe response and sampletemperature rise.
 15. The method of claim 14 comprising the step ofscaling the probe response at a plurality of wavelengths by thelinearization function.